1. Field of the Invention
The invention relates to a system and a method for measuring fatigue for mechanical components of an aircraft, for example an airplane, and a method for maintaining the aircraft.
Safety rules require airlines to monitor the fatigue of the components of the aircraft that they operate, these components being subjected to a large number of mechanical stresses (or loads). The components are therefore the subject of an overhaul (or maintenance) in a regular and recurrent manner.
2. Description of the Related Art
For example, the components for suspending the turbojets from the airplanes are subjected to strict inspections. Each overhaul of a suspension makes it necessary to stop the operation of the airplane and to remove the suspension in order to carry out the tests. The frequency of the overhauls is determined in advance and an overhaul is carried out systematically on expiration of each preset time period (for example every 2600 flight cycles (takeoff-flight-landing)), irrespective of the real state of fatigue of the component. So as not to take the risk of carrying out an overhaul too long after the appearance of a state of fatigue requiring an intervention such as a repair or a replacement, this time period must be chosen (by computation or empirically) as being the minimum period beyond which there is a risk that the component will break, even if this risk remains statistically marginal. This minimum period therefore corresponds to the situations of the components subjected to accidental stresses; accordingly, many overhauls are carried out on components that could have been used without danger for longer since they have not been subjected to accidental stresses. Finally, in the absence of analysis of the real stresses to which a component has been subjected, the worst case scenario is always taken with respect to the possible damaging of the latter, which leads to overhauls that are often premature.
Moreover, and for safety reasons, the components are used for a shorter period than they could actually be, so that they do not achieve periods of use during which the risk of breakage exceeds a certain threshold. Again, in the absence of analysis of real conditions of use of the components, criteria are chosen that correspond to the worst case scenarios and it is for this reason that conventionally in the aeronautics field a component is replaced halfway through its theoretical service life, irrespective of its real state of fatigue. The final effective profitability of the components (the ratio of the effective usage period of the component over its theoretical capacity) is therefore of the order of 50%, which it would be desirable to improve.
Moreover, because of the frequent overhauls (making it necessary to remove the suspension from an airplane and then to reinstall it on an airplane that is a priori different) and different service lives of the various components of an airplane, the monitoring of the service life of a suspension is complex. In particular, it may happen that the serial number etched on a metal suspension wears off over time; in this case, not being able to refer to its history and in order to take no risk, the estimate of its period of use must be made with the most pessimistic assumptions, for example considering that this suspension was installed in the first airplane fitted with this type of suspension and has flown continuously since then; in practice, the use of the component has been less than this pessimistic notional assumption such that the suspension will be replaced too soon.
Moreover, although there currently exist indirect indicators of the fatigue of suspensions, they can only be approximate and provide uncertain information. Thus, in order to estimate the state of fatigue of a suspension, data are sometimes used that are measured by the inertial unit of the airplane which determines whether the airplane has been subjected to exceptional stresses such as a hard landing; accordingly, a computation of load transfers from the inertial unit to the component is carried out. Nevertheless, although a hard landing may indeed impose exceptional loads on a suspension, it is not systematically the case and one may be induced to overhaul a suspension when the landing has not actually stressed the suspension, for example because the forces have been absorbed and have not been transmitted to the suspension.
The prior art has not truly dealt with the question of measuring the fatigue in order to anticipate the overhauls; it has rather concentrated on detecting the breakage of components, such as for example in patent application FR 2,923,540 in the name of the Applicant.